1. Field of the Invention
The present invention relates to a WC—Co system cemented carbide having high strength and high toughness and is excellent in wear resistance, toughness, chipping resistance and thermal crack resistance, and is also applied for tools for cold forging, rolls, bits for mining tool, crushing blades, cutter blades and wear resistant tools. The WC—Co system in the present invention means that it comprises not only hard grains composed mainly of WC and iron group metal powder containing Co, but also at least one kind selected from the group consisting of carbide, nitride, carbonitride and boride of elements in Groups IVa, Va and VIa of the Periodic Table, excluding WC, as hard grains.
2. Description of the Related Art
A commercially available wear resistant cemented carbide is a composite material of a WC hard phase and a Co metal phase, and is a typical one of a dispersion type alloy. Mechanical properties thereof depend on the grain size of the WC hard phase and the amount of a Co binder metal phase and, particularly, hardness and toughness are antinomic with each other. To fully make use of extremely excellent hardness of the cemented carbide, various proposals have been made on a cemented carbide having high strength and high toughness.
For example, Japanese Examined Patent Publication (Kokoku) No. 47-23049 discloses a high strength alloy comprising tungsten carbide plate-shaped grains having unequal sizes, wherein a maximum size is 50 μm or less and the maximum size is at least three times larger than a minimum size, and Fe group metal. However, the plate-shaped tungsten carbide having unequal sizes is hardly applied for various wear resistant cemented carbide products which require a near net shape because an oriented WC grain growth structure is obtained by applying a shear force through rolling while heating using a fine tungsten carbide as a starting material.
Furthermore, Japanese Unexamined Patent Publication (Kokai) No. 02-274827 relates to a technology for manufacturing an anisotropic cemented carbide compact having excellent crack propagation resistance or toughness and describes a method comprising the steps of oxidizing a cemented carbide, which has already sintered, followed by reduction and further carbonization to obtain a WC—Co mixed powder having anisotropy. However, it is a method using the used cemented carbide after regeneration and a leased facility is required, and therefore it is difficult to cope with such a problem.
These inventions relate to a method for producing a cemented carbide having high hardness and high toughness, which has entirely uniform structure, by employing a specific grain form such as anisotropy WC grains or plate crystal tungsten carbide as a hard phase. On the other hand, a method for producing a high strength cemented carbide as a composite material is also proposed.
Japanese Unexamined Patent Publication (Kokai) No. 08-127807 discloses a gradient composite material comprising the surface layer portion having a ceramic grain growth structure and the interior enriched with a metal phase, which is produced by impregnating with a grain growth accelerator from the surface of a compact and firing the compact after drying.
Furthermore, Japanese Patent Unexamined Publication (Kokai) No. 2002-249843 discloses that a composite material having high hardness, high strength and high toughness, which has a grain growth structure and a three-dimensional network structure in the surface layer portion, is obtained by forming a mixed powder of non-oxide ceramic grains and metal grains into a compact, and coating the surface of the compact with a boron compound-containing solution, followed by sintering. However, these proposals only make mention of toughening due to a rain growth structure of the surface layer portion and do not make no mention of the fact that the grain size of the surface layer portion is decreased than that of the inner portion.
On the other hand, Japanese Unexamined Patent Publication (Kokai) No. 04-128330 proposes a sintered alloy having gradient composition structure wherein the concentration of a binder phase gradually increases from the surface to the interior and also the mean grain size of a hard phase gradually increases, which is produce by coating a pressed compact made of a sintered alloy comprising a hard layer composed mainly of a metal carbide and a binder layer made of a ferrous metal before sintering with various diffusion elements, and subjecting to liquid phase sintering thereby reacting the diffusion element with the binder layer on the surface of the hard phase.    [Patent Document 1] Japanese Examined Patent Publication (Kokoku) No. 47-23049    [Patent Document 2] Japanese Unexamined Patent Publication (Kokai) No. 02-274827    [Patent Document 3] Japanese Unexamined Patent Publication (Kokai) No. 08-127807    [Patent Document 4] Japanese Patent Unexamined Publication (Kokai) No. 2002-249843    [Patent Document 5] Japanese Unexamined Patent Publication (Kokai) No. 04-128330